1. Field of the Invention
The present invention is directed to a composite element capable of generating voltage pulses when disposed in a magnetic field, and particularly to such a composite element suitable for use as a marker in an anti-theft security system.
2. Description of the Prior Art
A composite member for generating voltage pulses is described in German Patent No. 31 52 008 composed of a core and an envelope of two magnetic materials having different coercivity, with the core and envelope being mechanically stressed relative to one another by a thermal treatment. The hard magnetic part is composed of a Co-V-Fe alloy or of a Co-Cr-Fe alloy. For example, an alloy in the range of 45 through 55 weight percent cobalt, 30 through 50 weight percent iron and 4 through 14 weight percent (chrome + vanadium) is used for the magnetically hard material, whereas nickel is provided as the soft magnetic material.
This known composite member exists in the form of an elongated magnetic switch core.
German Published Application No. 2 933 337 also discloses a composite member composed of nickel or of unalloyed steel as a stressing component and a cobalt-vanadium-iron alloy as a magnetically active switch component. This composite member is employed for alarm systems. As described in this document, the goal is a high coercivity field strength of the switch components as a result of the iron-cobalt-vanadium alloy in order to be insensitive to disturbing fields. This requires a high detection field strength in alarm systems and thus limits the possible applications. Moreover, this known composite member cannot be deactivated by magnetic influencing.
An elongated composite member having a low response field strength of 1.0 Oe (about 0.8 A/cm) is also disclosed by U.S. Pat. No. 4,660,025 (Humphrey). For example, an elongated wire of amorphous material that is 7.6 cm long is used and it is recited therein that the length of this wire can be between 2.5 and 10 cm.
In this known security marker, however, short lengths of the marker can be realized only given an extraordinarily thin cross section, again resulting in an extremely low output pulse. The reason for this is that the shearing by the demagnetizing field, that is relatively large in comparison to the diameter given short marker lengths, does not permit an adequate Barkhausen discontinuity and thus an adequate sudden remagnetization, to be achieved. This amorphous security marker can likewise not be magnetically deactivated.
Extremely small diameters of these wires, however, have the disadvantage that -- for example, given employment for anti-theft security markers -- they cause only an extremely slight modification of the alternating field employed for triggering the alarm, so that correspondingly sensitive measuring instruments or greater lengths of the marker are required.